As Wen Jiabao, the Chinese Premier, sips tea with the Prime Minister at
Downing Street on Monday, both men will already know that their countries
are in a state of undeclared war.

Although the Cold War is long over, MI5 is left tackling a constant threat from Chinese espionage.

Jonathan Evans, the director general of MI5, has singled out China for their "unreconstructed attempts" to spy on Britain saying they "continue to devote considerable time and energy trying to steal our sensitive technology on civilian and military projects and trying to obtain political and economic intelligence at our expense."

MI5 believes that the Chinese government "represents one of the most significant espionage threats to the UK."

A 14-page "restricted" report from MI5's Centre for the Protection of National Infrastructure (CPNI) last year, described how China has attacked British defence, energy, communications and manufacturing companies in a concerted hacking campaign.

One of the main focuses of Britain's Office of Cyber Security and the Cyber Security Operations Centre based at GCHQ is the constant attempts to break into government departments and private sector companies by Chinese hackers.

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But the CPNI report also claimed that China has gone much further, saying Chinese agents are trying to cultivate "long-term relationships" with the employees of key British companies, adding: "Chinese intelligence services have also been known to exploit vulnerabilities such as sexual relationships and illegal activities to pressurise individuals to co-operate with them."

It warned that hotel rooms in major Chinese cities, such as Beijing and Shanghai, frequented by foreigners, were likely to be bugged and have been searched while the occupants are out of the room.

Victims include one of Gordon Brown's aides who was caught in a "honeytrap" operation by a suspected Chinese intelligence agent who is thought to have stolen his Blackberry phone after she approached him in a Shanghai hotel disco three years ago.

No doubt David Cameron's aides will have their wits about them throughout the visit.